This invention relates to an improved light-polarizing film and also to the production thereof.
A conventional light-polarizing film is produced by impregnating a film of polyvinyl alcohol with an aqueous solution of a light-polarizing element (such as iodine or a dichroic dye) or an agent forming a dichromic light-polarizing element and then thermally stretching the film several times the length so that high molecular micelles are unidirectionally oriented. However, the base material in this case is of a water-soluble high molecular weight substance, and therefore the resulting light-polarizing film is low in the resistance to water so that it deteriorates even with the moisture in air.
U.S. Pat. No. 4,025,688 teaches the manufacture of light polarizers by providing a film of polyvinyl alcohol with a dichroic stain such as iodine. The polarizer may optionally be supported by a layer of cellulose acetate butyrate bonded to the polarizer through a hydrophilic tie coat.
A problem with this type of polarizer is it demonstrates poor hydrolytic stability. That is, its polarizing efficiency, or ability to polarize light, decreases over time as heat and humidity increases. This tendency is disadvantageous for such uses as in liquid crystal displays especially for automobile instruments which must function in a wide range of heat and humidity conditions.
In the production of polarizing sheets, a film of a substrate is laminated to a film of oriented polyvinyl alcohol. This laminate is then passed through an aqueous staining bath containing iodine and one or more iodide salts to tint the polyvinyl alcohol film. Stabilizing treatments may then be employed.
The present invention provides an improved hydrolytically stabilized polarizer employing an optically advantageous, high moisture barrier fluorocarbon film substrate.